1. ## Simple Intrest

A sum of money doubles itself in 7 years.In how many years will it become 4 times at SI
(a) 15
(b)21
(c)18
(d)25

2. Call the sum of money x.

So if it doubles in 7 years then

After 7 years it becomes 2x

2x becomes 4x after another 7 years which is four times the original ammount.

So i would have said that the answer would be 14 years but maybe im missing something.

3. Originally Posted by Niall101
Call the sum of money x.

So if it doubles in 7 years then

After 7 years it becomes 2x

2x becomes 4x after another 7 years which is four times the original ammount.

So i would have said that the answer would be 14 years but maybe im missing something.
I also tried it the same way.But answer was given as 21.
Any way thank you.

4. ah yes i see the problem I used compound interest.

simple interest you do no get interest on the interest.

So if sum of money = x

Every 7 years you will get x interest

after 7 years you have x + x

After 14 years you have x + x + x

After 21 years you have x + x +x +x =4x